High purity tantalum metal and tantalum pentoxide have become increasingly important to the electronics industry in the preparation of advanced electronic materials used in the manufacture of devices such as surface acoustic wave filters, pyroelectric infrared sensors, optoelectronic devices and capacitors.
Prior to 1957, niobium and transition metals were separated from tantalum by a fractional crystallization process known as the Marignac process which exploits the difference in solubility between K2TaF7 and K2NbOF5.H2O and other fluorometallates. However, tantalum pentoxide obtained by this process contains high levels of impurities such as Nb (1000–3000 ppm), Si (up to 3000 ppm), Ti (up to 100 ppm), and Fe (up to 2000 ppm). These exceed the total allowable transition metal impurity level (less than 1000 ppm) in the tantalum wire used in the manufacture of capacitors.
In the late 1950″s, modern solvent extraction and ion exchange processes supplanted the use of the Marignac process. Examples of liquid-liquid solvent extraction and ion exchange methods are disclosed in U.S. Pat. Nos. 3,117,833, 3,712,939, 4,673,554 and 4,446,115. In a solvent extraction process, ore concentrates containing at least 25 wt. % tantalum and niobium pentoxide are decomposed chemically in hydrofluoric acid media and the dissolved tantalum and niobium species are separated from the residue by filtration. The filtrate containing tantalum (as TaF72−) and niobium (as NbOF52−) in an HF/H2SO4 medium is brought into contact with an organic phase, usually methyl iso-butyl ketone (MIBK), which selectively absorbs tantalum and niobium species leaving impurities such as titanium, iron, and silicon in the aqueous phase. Niobium is separated from tantalum by back extraction with sulfuric acid. Finally, tantalum (TaF72−) is eluted from the organic phase (MIBK) by an ammonium fluoride solution and converted into K2TaF7 by precipitation with KCl. Although Ta2O5 (or K2TaF7) made by the solvent extraction method can be used for most applications, this procedure is costly and uses large quantities of HF which requires special handling.